In HPLC samples are kept in containers (small bottles or microtiter plates) which are frequently sealed by so-called septa. To take a sample, a penetrating element is jabbed through the septa in order to be able to take up the sample through the hollow needle. On the one hand the penetrating element can be a sample needle, by means of which the sample is directly taken up, the needle piercing and contacting the septum. However, synonymous with the invention, the penetrating element can also be a so-called pricker which, in the form of a needle with a larger inside diameter, penetrates the septum and comprises in its interior a free channel for the friction-free passage of a sample needle. The pricker prepares the path for the sample needle into the container as it were and protects it against contact with the septum or in general the container covering. Whilst in this case it is possible to withdraw the needle itself in a friction-free manner out of the container, the pricker rubs against the septum. A moving unit moves the pricker up and down and carries a further moving unit which can move or at least guide the sample needle relative to the pricker.
When the penetrating element is subsequently withdrawn from the container it must be ensured that the container is held in its position in spite of the friction forces between the penetrating element and the septum. Otherwise it would be conceivable for a penetrating element, which is to be withdrawn upward from the container, also to raise the container by means of the friction forces, which could result in collisions between the sample vessel and other vessels or devices of the sampler.
In order to avoid this, the prior art makes known hold-down devices which act upon the top surface of the container or of the septum with a holding force whilst the penetrating element is pulled out. The hold-down device can be moved up and down by means of an own drive where applicable. As an alternative to this, a sprung coupling with the sampling unit which moves the penetrating element is also known. In this case, the hold-down device is pressed onto the container at an increasing spring force whilst the penetrating element penetrates the septum. When the movement of the sampling unit is in the opposite direction, the spring force must be great enough to enable the penetrating element to be pulled out of the container completely before the spring force weakens too much and the hold-down device together with the sampling unit is able to move upwards again.
As an alternative to this, cascaded drives are also known where the drive of the sampling unit is moved together with the hold-down device by means of a hold-down device drive. As soon as the hold-down device is placed in position and consequently fixes the sample vessel, the drive is actuated in order to move the penetrating element into the container and then out again. The hold-down device can then be raised from the vessel together with the sampling unit it is carrying.
The aforementioned structures are expensive and the requirements are only unsatisfactorily met. Cascaded drives require increased expenditure on structure and control. In the case of the spring-preloaded hold-down devices, to pierce the penetrating element the spring force always has to be countered, which loads the drives unnecessarily. In addition, the spring force changes in dependence on the depth of insertion of the needle or of the pricker. As a rule, very long compression or tension springs are used for a hold-down force that is as uniform as possible, which requires increased installation space. In addition, such compression springs can buckle or scrape against their guides.